78 
CiAccio^ on the Nerves of the Cornea. 
agents, it is impossible for the nerves to be seen. But by 
the use of chemical agents the natural aspect of the nerves 
is always altered, and if we are not very careful in using 
them, we may destroy the finest fibres. 
3. The third difficulty arises from the peculiar structure 
of the cornea itself, which contains a very large number of 
corpuscles, with many anastomosing processes. This un- 
doubtedly causes much difficulty in tracing the ultimate 
nerve-fibres running through its lamellated structure ; and 
if we do not use much diligence in observing, we may mis- 
take the processes of the cornea-corpuscles for the finest 
nerve-fibres, and draw the erroneous conclusion that there 
exists an intimate connexion between the nerve-fibres and the 
cornea-corpuscles. 
It seems to me that these three difficulties which I have 
mentioned, if not totally, can at least in part be surmounted. 
In fact, if we select for the microscopical investigation those 
animals iu which the cornea is not thick, we shall find that 
the first difficulty decreases in proportion as the thickness of 
the cornea which we have to examine diminishes. Hence 
the cornea of small birds, of the frog, mouse, and so on, are 
more suitable for investigation than that of man or the larger 
animals. I have found by experience that in the sparrow's 
cornea the nerves can be easily seen and traced for a very 
long distance, because in this little bird the distribution of 
nerves is more simple than in the mouse and frog, and the 
thickness of its cornea is such that by only dividing it trans- 
versely we are enabled to examine it with high powers. We 
cannot, in my opinion, completely overcome the second diffi- 
culty in the present state of our knowledge ; but the only 
thing we can do is to moderate, in some way, the chemical 
action of those agents which we are obliged to employ for 
bringing out the nerves, which lie hidden among the proper 
fibrous tissue of the cornea. In such a case the best way is 
to employ only a small quantity of the reagent, because, if 
required, we can always add more ; while, on the contrary, 
we can never remedy the harm produced by a large quantity. 
The third difficulty can only be avoided if we are careful in 
observing. The processes of the cornea- corpuscles generally 
exhibit such an appearance that they are with great difficulty 
distinguished from the finest nerve-fibres. Like the nerve- 
fibres, they become granular by the action of acetic acid. 
How much this increases the difficulty it is hardly necessary 
to say. In no other way, therefore, we can distinguish the 
processes of the cornea-corpuscles from the finest nerve-fibres, 
but by following both to their respective origins, viz., the 
